Polyimides exhibit an attractive combination of thermal stability (>500° C.), mechanical toughness and chemical resistance, in addition to having excellent dielectric properties. Because of their high degree of ductility and inherently low CTE, polyimides can be readily implemented into a variety of microelectronic applications.
Polyimide films are frequently used as passivation layers for silicon wafers. Polyimide passivation layers are typically 4-6 microns in thickness, and protect the delicate thin films of metal and oxides on the chip surface from damage during handling and from induced stress after encapsulation in plastic molding compound. Patterning is simple and straightforward. Because of the low defect density and robust plasma etch resistance inherent with polyimide films, a “single mask” process can be implemented, which permits the polyimide layer to function both as a stress buffer and as a dry etch mask for an underlying silicon nitride layer. In addition, polyimide layers have been readily used for flip chip bonding applications, including both C-4 and dual-layer bond pad redistribution (BPR) applications. Polyimide layers can be patterned to form the structural components in microelectromechanical systems (MEMS).
Polyimides may also serve as an interlayer dielectric in both semiconductors and thin film multichip modules (MCM-D's). The low dielectric constant, low stress, high modulus, and inherent ductility of polyimide films make them well suited for these multiple layer applications. Other uses for polyimides include alignment and/or dielectric layers for displays, and as a structural layer in micromachining applications. In lithium-ion battery technology, polyimide films can be used as protective layers for PTC thermistor (positive temperature coefficient) controllers.
In the fabrication of microelectronic devices, polyimides are typically applied as a solution of the corresponding polyamic acid precursors onto a substrate, and then thermally cured into a smooth, rigid, intractable polymeric film or structural layer. The film can be patterned using a lithographic (photographic) process in conjunction with liquid photoresists. Typically, polyimides are formed in situ through cyclodehydration of the polyamic acid precursors. This imidization step also requires the evaporation of high boiling, polar aprotic solvents, which can be difficult to drive away as the polyimide is formed.
Existing polyimide passivation materials generate a high degree of stress on the wafer. This stress is known to cause delamination of the passivation material. Moreover, as silicon wafers have become thinner, it has been found that the polyimides used for passivation layers tend to warp the wafer upon thermal cure, resulting in a concave or convex wafer surface. This phenomenon creates a variety of problems for the semiconductor fabrication and packaging industry.
Conventional polyimide passivation materials are generally hydrophilic and usually require tedious multi-step processes to form the vias required for electrical interconnects. For example, polyimide materials have been used as interlayer dielectric materials in microelectronic devices such as integrated circuits (IC's) due to their having a dielectric constant that is lower than that of silicon dioxide. Also, such polyimide materials can serve as a planarization layer for IC's as they are generally applied in a liquid form, allowed to level, and subsequently cured. However, polyimide materials readily absorb moisture even after curing and this absorption can result in device failure. In addition, polyimides are generally not easily patterned as is often required in the manufacture of IC's and other microelectronic devices.
Accordingly, there is a need for hydrophobic, low modulus polyimides that are compatible with very thin silicon wafers, i.e., polyimide passivation layers that will not warp thin silicon wafers.